Hi there,
You’ll need a bit of coding to achieve this.
You can override the template files activity/index.php and groups/single/activity.php. And in those files, you can roll out your own filters to conditionally load actiivty-post-form template.
e.g: instead of if ( is_user_logged_in() && bp_group_is_member() )
, you can use if ( is_user_logged_in() && bp_group_is_member() && bp_group_is_admin() )
That should get you started in right direction.